How to Call a JavaScript Function Using onclick Event

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Nithin Krishnan Feb 02, 2024
JavaScript
  1. Calling JavaScript Function Using the onclick Event Handler
  2. Attaching a JavaScript Function to an HTML Element Using Pure JavaScript
  3. Defining a Function on the onclick Event Handler in the HTML Element
  4. Calling a JavaScript Function Using addEventListener
How to Call a JavaScript Function Using onclick Event

Even if we design static HTML content for a webpage, we may want to make it user interactive. HTML elements, by default, are dumb. The <a> tag is user interactive but is mainly used to load a specific path. HTML elements like div can be made user interactive by adding an event handler to them. We will be discussing one such event handler called the onclick event handler. There are a couple of ways in which we can add it to an HTML element.

Calling JavaScript Function Using the onclick Event Handler

We can bind a JavaScript function to a div using the onclick event handler in the HTML or attaching the event handler in JavaScript. Let us refer to the following code in which we attach the event handler to a div element.

<div id="interactive-div" onclick="changeMyColor('interactive-div')" style="cursor: pointer;">Click me</div>

function changeMyColor(id) {
  var el = document.getElementById(id);
  el.style.color = 'blue';
}

The div element does not accept any click events by default. In the code, we bind a JavaScript function changeMyColor() to the onclick event handler of the div. It now makes the div clickable. Even though we define an onclick event handler to a div, still the user will not know if the div accepts a click or not unless he clicks. It is a good UI/UX to have the cursor marked differently once the user hovers on our target div. For this purpose, we assign an inline style style="cursor: pointer;" to the div element. Once the user hovers the div, the cursor will show a hand sign indicating it as clickable.

function changeMyColor(id) {
  var el = document.getElementById(id);
  el.style.color = 'blue';
}

In the JavaScript code, we define the function changeMyColor() passed to the onclick event of the div. The function takes the id of the div as a parameter. It selects the element using the document.getElementById(id) and changes the font color of the element to blue using the line el.style.color = "blue";. Hence, we assigned a JavaScript function to the div and made it clickable.

Attaching a JavaScript Function to an HTML Element Using Pure JavaScript

In the last approach, we declared the onclick event handler in the HTML. There is a way to keep the HTML clean and attach a JavaScript function to the div or the corresponding HTML element without specifying any onclick event handler in the HTML. Here, we assign the click event in JavaScript rather than using HTML. Hence, this approach is purely JavaScript-based and keeps the HTML clean. Such a style of programming is called Unobtrusive JavaScript. Refer to the following code to understand the concept better.

<div id="interactive-div" style="cursor: pointer;">Click me</div>

window.onload =
    function() {
  var el = document.getElementById('interactive-div');
  el.onclick = changeMyColor;
}

function changeMyColor() {
  var el = document.getElementById('interactive-div');
  el.style.color = 'blue';
}

Referring to the above code, we have kept the HTML similar to the previous code sample. We did not add any onclick event handler in the HTML. Instead, we defined the onclick event handler in JavaScript and attached the same function changeMyColor in the JavaScript code. There are a couple of things to note here:

  • We attach the onclick event listener inside the window.onload. If we do not use the window.onload method, it will throw an error saying Uncaught TypeError: Cannot set property 'onclick' of null. This happens because, the document.getElementById("interactive-div") tries to query for the element with id interactive-div while the HTML is loading. As the <script> tag comes in the <head> section, the JavaScript code executes the <script> tag contents before actually loading the div that resides in the <body> tag. Hence, the getElementById() function will not find the element and returns null. Hence it spills out an error saying that it cannot set onclick of null.
  • Another point to note is that we assign the function to the onclick event handler without the parenthesis (). If we write el.onclick = changeMyColor(); instead of el.onclick = changeMyColor;, it will execute the function on load of the HTML which is not what we want. Hence, we pass the function reference of changeMyColor to the onclick event handler. Once the user triggers the onclick event by clicking on the div, the function call changeMyColor happens. And the div turns blue.

Defining a Function on the onclick Event Handler in the HTML Element

There are a few more ways to call a JavaScript function on the onclick event. We can define the method in HTML. But it is not a recommended way as it makes the HTML untidy. Refer to the following code to understand better.

<div onclick="(function() { alert('hello there'); })()" style="cursor: pointer;">click me!</div>

On clicking the div, the above code will popup the hello there text. Here, we code the function definition inline in the HTML. It is an anonymous inline method. And we cannot use the same function elsewhere as it does not have an associated name. Lengthier function definitions will be cumbersome and make the code readability poor.

Calling a JavaScript Function Using addEventListener

One way to add the onclick event handler was to use the .onclick() function. There is another way that is more generic and can be used to add many more event listeners. For that we use .addEventListener() function of JavaScript. This function can be used on an element, document interface or even a window object. We can use the .addEventListener() function to set up a method invocation when a particular event is triggered.

  • Parameters: It takes a couple of mandatory parameters: the type of an event, for example, click etc., and the listener function. The listener function gets executed when the event triggers.
  • Usage: Refer to the following code.
<div style="cursor: pointer;" id="interactive-div">click me!</div>

document.addEventListener('DOMContentLoaded', function() {
  document.getElementById('interactive-div')
      .addEventListener('click', function() {
        alert('hello');
      });
});

This coding style is purely JavaScript-based unobtrusive JavaScript programming. Here, we have added the click listener using the .addEventListener() function and for this to work, we will have to make sure the .getElementById("interactive-div") returns the element. Hence we wrap the code in the DOMContentLoaded event listener in the document interface. Else, the code will throw an error saying that it cannot add event to a null object.